diff --git a/libcontainer/configs/validate/rootless.go b/libcontainer/configs/validate/rootless.go
index 6d32704a32a..faceea04c09 100644
--- a/libcontainer/configs/validate/rootless.go
+++ b/libcontainer/configs/validate/rootless.go
@@ -2,6 +2,7 @@ package validate
 
 import (
 	"errors"
+	"fmt"
 	"strconv"
 	"strings"
 
@@ -28,25 +29,18 @@ func rootlessEUIDCheck(config *configs.Config) error {
 	return nil
 }
 
-func hasIDMapping(id int, mappings []configs.IDMap) bool {
-	for _, m := range mappings {
-		if id >= m.ContainerID && id < m.ContainerID+m.Size {
-			return true
-		}
-	}
-	return false
-}
-
 func rootlessEUIDMappings(config *configs.Config) error {
 	if !config.Namespaces.Contains(configs.NEWUSER) {
 		return errors.New("rootless container requires user namespaces")
 	}
-
-	if len(config.UIDMappings) == 0 {
-		return errors.New("rootless containers requires at least one UID mapping")
-	}
-	if len(config.GIDMappings) == 0 {
-		return errors.New("rootless containers requires at least one GID mapping")
+	// We only require mappings if we are not joining another userns.
+	if path := config.Namespaces.PathOf(configs.NEWUSER); path == "" {
+		if len(config.UIDMappings) == 0 {
+			return errors.New("rootless containers requires at least one UID mapping")
+		}
+		if len(config.GIDMappings) == 0 {
+			return errors.New("rootless containers requires at least one GID mapping")
+		}
 	}
 	return nil
 }
@@ -68,8 +62,8 @@ func rootlessEUIDMount(config *configs.Config) error {
 					// Ignore unknown mount options.
 					continue
 				}
-				if !hasIDMapping(uid, config.UIDMappings) {
-					return errors.New("cannot specify uid= mount options for unmapped uid in rootless containers")
+				if _, err := config.HostUID(uid); err != nil {
+					return fmt.Errorf("cannot specify uid=%d mount option for rootless container: %w", uid, err)
 				}
 			}
 
@@ -79,8 +73,8 @@ func rootlessEUIDMount(config *configs.Config) error {
 					// Ignore unknown mount options.
 					continue
 				}
-				if !hasIDMapping(gid, config.GIDMappings) {
-					return errors.New("cannot specify gid= mount options for unmapped gid in rootless containers")
+				if _, err := config.HostGID(gid); err != nil {
+					return fmt.Errorf("cannot specify gid=%d mount option for rootless container: %w", gid, err)
 				}
 			}
 		}
diff --git a/libcontainer/specconv/spec_linux.go b/libcontainer/specconv/spec_linux.go
index c1632ac12ad..2529e1e7e29 100644
--- a/libcontainer/specconv/spec_linux.go
+++ b/libcontainer/specconv/spec_linux.go
@@ -18,6 +18,7 @@ import (
 	"github.com/opencontainers/runc/libcontainer/configs"
 	"github.com/opencontainers/runc/libcontainer/devices"
 	"github.com/opencontainers/runc/libcontainer/seccomp"
+	"github.com/opencontainers/runc/libcontainer/userns"
 	libcontainerUtils "github.com/opencontainers/runc/libcontainer/utils"
 	"github.com/opencontainers/runtime-spec/specs-go"
 	"github.com/sirupsen/logrus"
@@ -968,6 +969,21 @@ func setupUserNamespace(spec *specs.Spec, config *configs.Config) error {
 		config.UIDMappings = toConfigIDMap(spec.Linux.UIDMappings)
 		config.GIDMappings = toConfigIDMap(spec.Linux.GIDMappings)
 	}
+	if path := config.Namespaces.PathOf(configs.NEWUSER); path != "" {
+		// Cache the current userns mappings in our configuration, so that we
+		// can calculate uid and gid mappings within runc. These mappings are
+		// never used for configuring the container if the path is set.
+		uidMap, gidMap, err := userns.GetUserNamespaceMappings(path)
+		if err != nil {
+			return fmt.Errorf("failed to cache mappings for userns: %w", err)
+		}
+		config.UIDMappings = uidMap
+		config.GIDMappings = gidMap
+		logrus.WithFields(logrus.Fields{
+			"uid_map": uidMap,
+			"gid_map": gidMap,
+		}).Debugf("config uses path-based userns configuration -- current uid and gid mappings cached")
+	}
 	rootUID, err := config.HostRootUID()
 	if err != nil {
 		return err
diff --git a/libcontainer/userns/userns_maps.c b/libcontainer/userns/userns_maps.c
new file mode 100644
index 00000000000..84f2c6188c3
--- /dev/null
+++ b/libcontainer/userns/userns_maps.c
@@ -0,0 +1,79 @@
+#define _GNU_SOURCE
+#include <fcntl.h>
+#include <sched.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <stdarg.h>
+#include <stdlib.h>
+
+/*
+ * All of the code here is run inside an aync-signal-safe context, so we need
+ * to be careful to not call any functions that could cause issues. In theory,
+ * since we are a Go program, there are fewer restrictions in practice, it's
+ * better to be safe than sorry.
+ *
+ * The only exception is exit, which we need to call to make sure we don't
+ * return into runc.
+ */
+
+void bail(int pipefd, const char *fmt, ...)
+{
+	va_list args;
+
+	va_start(args, fmt);
+	vdprintf(pipefd, fmt, args);
+	va_end(args);
+
+	exit(1);
+}
+
+int spawn_userns_cat(char *userns_path, char *path, int outfd, int errfd)
+{
+	char buffer[4096] = { 0 };
+
+	pid_t child = fork();
+	if (child != 0)
+		return child;
+	/* in child */
+
+	/* Join the target userns. */
+	int nsfd = open(userns_path, O_RDONLY);
+	if (nsfd < 0)
+		bail(errfd, "open userns path %s failed: %m", userns_path);
+
+	int err = setns(nsfd, CLONE_NEWUSER);
+	if (err < 0)
+		bail(errfd, "setns %s failed: %m", userns_path);
+
+	close(nsfd);
+
+	/* Pipe the requested file contents. */
+	int fd = open(path, O_RDONLY);
+	if (fd < 0)
+		bail(errfd, "open %s in userns %s failed: %m", path, userns_path);
+
+	int nread, ntotal = 0;
+	while ((nread = read(fd, buffer, sizeof(buffer))) != 0) {
+		if (nread < 0)
+			bail(errfd, "read bytes from %s failed (after %d total bytes read): %m", path, ntotal);
+		ntotal += nread;
+
+		int nwritten = 0;
+		while (nwritten < nread) {
+			int n = write(outfd, buffer, nread - nwritten);
+			if (n < 0)
+				bail(errfd, "write %d bytes from %s failed (after %d bytes written): %m",
+				     nread - nwritten, path, nwritten);
+			nwritten += n;
+		}
+		if (nread != nwritten)
+			bail(errfd, "mismatch for bytes read and written: %d read != %d written", nread, nwritten);
+	}
+
+	close(fd);
+	close(outfd);
+	close(errfd);
+
+	/* We must exit here, otherwise we would return into a forked runc. */
+	exit(0);
+}
diff --git a/libcontainer/userns/userns_maps_linux.go b/libcontainer/userns/userns_maps_linux.go
new file mode 100644
index 00000000000..d81290de2cb
--- /dev/null
+++ b/libcontainer/userns/userns_maps_linux.go
@@ -0,0 +1,171 @@
+//go:build linux
+
+package userns
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"unsafe"
+
+	"github.com/opencontainers/runc/libcontainer/configs"
+	"github.com/sirupsen/logrus"
+)
+
+/*
+#include <stdlib.h>
+extern int spawn_userns_cat(char *userns_path, char *path, int outfd, int errfd);
+*/
+import "C"
+
+func parseIdmapData(data []byte) (ms []configs.IDMap, err error) {
+	scanner := bufio.NewScanner(bytes.NewReader(data))
+	for scanner.Scan() {
+		var m configs.IDMap
+		line := scanner.Text()
+		if _, err := fmt.Sscanf(line, "%d %d %d", &m.ContainerID, &m.HostID, &m.Size); err != nil {
+			return nil, fmt.Errorf("parsing id map failed: invalid format in line %q: %w", line, err)
+		}
+		ms = append(ms, m)
+	}
+	if err := scanner.Err(); err != nil {
+		return nil, fmt.Errorf("parsing id map failed: %w", err)
+	}
+	return ms, nil
+}
+
+// Do something equivalent to nsenter --user=<nsPath> cat <path>, but more
+// efficiently. Returns the contents of the requested file from within the user
+// namespace.
+func spawnUserNamespaceCat(nsPath string, path string) ([]byte, error) {
+	rdr, wtr, err := os.Pipe()
+	if err != nil {
+		return nil, fmt.Errorf("create pipe for userns spawn failed: %w", err)
+	}
+	defer rdr.Close()
+	defer wtr.Close()
+
+	errRdr, errWtr, err := os.Pipe()
+	if err != nil {
+		return nil, fmt.Errorf("create error pipe for userns spawn failed: %w", err)
+	}
+	defer errRdr.Close()
+	defer errWtr.Close()
+
+	cNsPath := C.CString(nsPath)
+	defer C.free(unsafe.Pointer(cNsPath))
+	cPath := C.CString(path)
+	defer C.free(unsafe.Pointer(cPath))
+
+	childPid := C.spawn_userns_cat(cNsPath, cPath, C.int(wtr.Fd()), C.int(errWtr.Fd()))
+
+	if childPid < 0 {
+		return nil, fmt.Errorf("failed to spawn fork for userns")
+	} else if childPid == 0 {
+		// this should never happen
+		panic("runc executing inside fork child -- unsafe state!")
+	}
+
+	// We are in the parent -- close the write end of the pipe before reading.
+	wtr.Close()
+	output, err := io.ReadAll(rdr)
+	rdr.Close()
+	if err != nil {
+		return nil, fmt.Errorf("reading from userns spawn failed: %w", err)
+	}
+
+	// Ditto for the error pipe.
+	errWtr.Close()
+	errOutput, err := io.ReadAll(errRdr)
+	errRdr.Close()
+	if err != nil {
+		return nil, fmt.Errorf("reading from userns spawn error pipe failed: %w", err)
+	}
+	errOutput = bytes.TrimSpace(errOutput)
+
+	// Clean up the child.
+	child, err := os.FindProcess(int(childPid))
+	if err != nil {
+		return nil, fmt.Errorf("could not find userns spawn process: %w", err)
+	}
+	state, err := child.Wait()
+	if err != nil {
+		return nil, fmt.Errorf("failed to wait for userns spawn process: %w", err)
+	}
+	if !state.Success() {
+		errStr := string(errOutput)
+		if errStr == "" {
+			errStr = fmt.Sprintf("unknown error (status code %d)", state.ExitCode())
+		}
+		return nil, fmt.Errorf("userns spawn: %s", errStr)
+	} else if len(errOutput) > 0 {
+		// We can just ignore weird output in the error pipe if the process
+		// didn't bail(), but for completeness output for debugging.
+		logrus.Debugf("userns spawn succeeded but unexpected error message found: %s", string(errOutput))
+	}
+	// The subprocess succeeded, return whatever it wrote to the pipe.
+	return output, nil
+}
+
+func GetUserNamespaceMappings(nsPath string) (uidMap, gidMap []configs.IDMap, err error) {
+	var (
+		pid         int
+		extra       rune
+		tryFastPath bool
+	)
+
+	// nsPath is usually of the form /proc/<pid>/ns/user, which means that we
+	// already have a pid that is part of the user namespace and thus we can
+	// just use the pid to read from /proc/<pid>/*id_map.
+	//
+	// Note that Sscanf doesn't consume the whole input, so we check for any
+	// trailing data with %c. That way, we can be sure the pattern matched
+	// /proc/$pid/ns/user _exactly_ iff n === 1.
+	if n, _ := fmt.Sscanf(nsPath, "/proc/%d/ns/user%c", &pid, &extra); n == 1 {
+		tryFastPath = pid > 0
+	}
+
+	for _, mapType := range []struct {
+		name  string
+		idMap *[]configs.IDMap
+	}{
+		{"uid_map", &uidMap},
+		{"gid_map", &gidMap},
+	} {
+		var mapData []byte
+
+		if tryFastPath {
+			path := fmt.Sprintf("/proc/%d/%s", pid, mapType.name)
+			data, err := os.ReadFile(path)
+			if err != nil {
+				// Do not error out here -- we need to try the slow path if the
+				// fast path failed.
+				logrus.Debugf("failed to use fast path to read %s from userns %s (error: %s), falling back to slow userns-join path", mapType.name, nsPath, err)
+			} else {
+				mapData = data
+			}
+		} else {
+			logrus.Debugf("cannot use fast path to read %s from userns %s, falling back to slow userns-join path", mapType.name, nsPath)
+		}
+
+		if mapData == nil {
+			// We have to actually join the namespace if we cannot take the
+			// fast path. The path is resolved with respect to the child
+			// process, so just use /proc/self.
+			data, err := spawnUserNamespaceCat(nsPath, "/proc/self/"+mapType.name)
+			if err != nil {
+				return nil, nil, err
+			}
+			mapData = data
+		}
+		idMap, err := parseIdmapData(mapData)
+		if err != nil {
+			return nil, nil, fmt.Errorf("failed to parse %s of userns %s: %w", mapType.name, nsPath, err)
+		}
+		*mapType.idMap = idMap
+	}
+
+	return uidMap, gidMap, nil
+}
diff --git a/tests/integration/userns.bats b/tests/integration/userns.bats
index d52444cacc5..947681c9b7c 100644
--- a/tests/integration/userns.bats
+++ b/tests/integration/userns.bats
@@ -15,15 +15,25 @@ function setup() {
 	mkdir -p rootfs/{proc,sys,tmp}
 	mkdir -p rootfs/tmp/mount-{1,2}
 
+	to_umount_list="$(mktemp "$BATS_RUN_TMPDIR/userns-mounts.XXXXXX")"
 	if [ $EUID -eq 0 ]; then
 		update_config ' .linux.namespaces += [{"type": "user"}]
 			| .linux.uidMappings += [{"hostID": 100000, "containerID": 0, "size": 65534}]
-			| .linux.gidMappings += [{"hostID": 100000, "containerID": 0, "size": 65534}] '
+			| .linux.gidMappings += [{"hostID": 200000, "containerID": 0, "size": 65534}] '
 	fi
 }
 
 function teardown() {
 	teardown_bundle
+
+	if [ -v to_umount_list ]; then
+		while read -r mount_path; do
+			umount -l "$mount_path" || :
+			rm -f "$mount_path"
+		done <"$to_umount_list"
+		rm -f "$to_umount_list"
+		unset to_umount_list
+	fi
 }
 
 @test "userns with simple mount" {
@@ -83,3 +93,74 @@ function teardown() {
 	runc exec test_busybox cat /sys/fs/cgroup/{pids,memory}/tasks
 	[ "$status" -eq 0 ]
 }
+
+@test "userns join other container userns" {
+	# Create a detached container with the id-mapping we want.
+	update_config '.process.args = ["sleep", "infinity"]'
+	runc run -d --console-socket "$CONSOLE_SOCKET" target_userns
+	[ "$status" -eq 0 ]
+
+	# Configure our container to attach to the first container's userns.
+	target_pid="$(__runc state target_userns | jq .pid)"
+	update_config '.linux.namespaces |= map(if .type == "user" then (.path = "/proc/'"$target_pid"'/ns/" + .type) else . end)
+		| del(.linux.uidMappings)
+		| del(.linux.gidMappings)'
+	runc run -d --console-socket "$CONSOLE_SOCKET" in_userns
+	[ "$status" -eq 0 ]
+
+	runc exec in_userns cat /proc/self/uid_map
+	[ "$status" -eq 0 ]
+	if [ $EUID -eq 0 ]; then
+		grep -E '^\s+0\s+100000\s+65534$' <<<"$output"
+	else
+		grep -E '^\s+0\s+'$EUID'\s+1$' <<<"$output"
+	fi
+
+	runc exec in_userns cat /proc/self/gid_map
+	[ "$status" -eq 0 ]
+	if [ $EUID -eq 0 ]; then
+		grep -E '^\s+0\s+200000\s+65534$' <<<"$output"
+	else
+		grep -E '^\s+0\s+'$EUID'\s+1$' <<<"$output"
+	fi
+}
+
+@test "userns join other container userns [bind-mounted nsfd]" {
+	requires root
+
+	# Create a detached container with the id-mapping we want.
+	update_config '.process.args = ["sleep", "infinity"]'
+	runc run -d --console-socket "$CONSOLE_SOCKET" target_userns
+	[ "$status" -eq 0 ]
+
+	# Bind-mount the first containers userns nsfd to a different path, to
+	# exercise the non-fast-path (where runc has to join the userns to get the
+	# mappings).
+	target_pid="$(__runc state target_userns | jq .pid)"
+	userns_path=$(mktemp "$BATS_RUN_TMPDIR/userns.XXXXXX")
+	mount --bind "/proc/$target_pid/ns/user" "$userns_path"
+	echo "$userns_path" >>"$to_umount_list"
+
+	# Configure our container to attach to the first container's userns.
+	update_config '.linux.namespaces |= map(if .type == "user" then (.path = "'"$userns_path"'") else . end)
+		| del(.linux.uidMappings)
+		| del(.linux.gidMappings)'
+	runc run -d --console-socket "$CONSOLE_SOCKET" in_userns
+	[ "$status" -eq 0 ]
+
+	runc exec in_userns cat /proc/self/uid_map
+	[ "$status" -eq 0 ]
+	if [ $EUID -eq 0 ]; then
+		grep -E '^\s+0\s+100000\s+65534$' <<<"$output"
+	else
+		grep -E '^\s+0\s+'$EUID'\s+1$' <<<"$output"
+	fi
+
+	runc exec in_userns cat /proc/self/gid_map
+	[ "$status" -eq 0 ]
+	if [ $EUID -eq 0 ]; then
+		grep -E '^\s+0\s+200000\s+65534$' <<<"$output"
+	else
+		grep -E '^\s+0\s+'$EUID'\s+1$' <<<"$output"
+	fi
+}